1. Field of the Invention
The present invention relates to back-up air gap surge arresters, and more particularly, to an apparatus for providing a thermally sensitive breakdown path, a parallel back-up air gap surge arrester adapted to be affixed to a gas filled surge arrester in order to provide protection for telephone equipment from electrical overload.
2. Discussion of the Relevant Art
Numerous devices have been used to protect communication lines, such as those utilized by telephone companies and the like to protect against electrical overloads by surge arresters which are connected between the line and ground. The old carbon block surge arresters which were in use for many years have been replaced with gas filled surge arresters which comprise a pair of electrodes separated by an air gap maintained in a gas filled atmosphere. With a lightning strike, for example, or a voltage overload, the gap between the electrodes is bridged temporarily is dissipated. In this manner, electrical circuits connected to telephone lines are protected.
The electrodes of gas filled surge arresters are commonly spaced 40 mils apart, as compared to the 3 mils of air gap space in a carbon block surge arrester. However, the gas at a subatmospheric pressure, is permanently sealed within the ceramic insulator surrounding the electrodes and imparts the electrical design characteristic to the device.
The DC breakdown voltage of a typical gas filled surge arrester is approximately 400 volts. However, should the sealed ceramic insulated tube become broken, permitting an air leak between the electrodes, the DC breakdown voltage of the gas filled surge arrester, now vented to the air becomes considerably higher, in the vicinity of 33,500 volts. This breakdown voltage is much too high to adequately protect the communication circuits. For this reason, telephone companies have required that a back-up or alternative air gap type surge arrester be installed in parallel with the gas filled surge arrester. An air gap surge arrester having a 3 mil gap spacing typically has a DC breakdown voltage of approximately 600 to 1000 volts. Thus, the combination of different types of arresters in parallel assures that the DC breakdown voltage will never exceed that of the air gap surge arrester, should the gas within the surge arrester escape.
Many different configurations have been utilized to accomplish a back-up surge protector such as the type which utilize an insulator having apertures provided therein so that the air gap has a length of approximately the thickness of the insulator and serves as a second arrester to provide the back-up protection. This configuration can produce an assembly which fits into the standard retaining cup, however it does not prove adequate to meet the electrical performance specifications desired by the telephone companies.
Another approach to the problem is disclosed in U.S. Pat. No. 4,340,923 issued to A. Bazarian, et al. on July 20, 1982. The configuration disclosed therein provides an additional electrode and a ceramic washer with a predetermined air gap placed therein. However, this device is not readily assembled and requires precise assembly technique.
Therefore, it is an object of the present invention to overcome the shortcomings of the prior art and provide a back-up air gap surge arrester which may be readily assembled and tested and thereafter affixed to a conventional gas surge arrester.
It is another object of the present invention to provide a circuit protector wherein the gas filled and air gap surge arrester share a common electrode.
It is still a further object of the present invention to provide a circuit protector which includes an air gap as well as a thermally sensitive insulator capable of protecting a circuit for overvoltage and overheating.